Steel sheet for vitreous enameling and production method

ABSTRACT

To provide a non-aging steel sheet for vitreous enameling, the steel sheet being excellent in resistance to bubbles and black spots, without the employment of decarbonization and denitrification annealing that raises the cost of production and also without the addition of expensive elements such as Nb, Ti, etc. that raise the cost of alloys. The steel sheet comprises, in mass, to C: 0.0050% or less, Si: 0.50% or less, Mn: 0.005 to 1.0%, P: 10×(B−11/14×N) to 0.10%, S: 0.080% or less, Al: 0.050% or less, N: 0.0005 to 0.020%, B: 0.60×N to 0.020%, and O: 0.002 to 0.0800%, and the shape of B nitrides is controlled mainly by adjusting hot-rolling conditions.

TECHNICAL FIELD

The present invention relates to a steel sheet for vitreous enameling,the steel sheet being excellent in enameling properties, workability andaging properties, and a method for producing the steel sheet at a lowcost.

BACKGROUND ART

A steel sheet for vitreous enameling was conventionally produced byapplying an annealing treatment for decarbonization and denitrificationand lowering C and N contained therein to several tens of ppm or less.However, such an annealing treatment for decarbonization anddenitrification had the drawbacks of low productivity and a highproduction cost. As a technology for avoiding an annealing treatment fordecarbonization and denitrification, Japanese Unexamined PatentPublication No. H6-122938 discloses a steel sheet for vitreousenameling, the steel sheet being produced from ultra-low carbon steelobtained by lowering the C content to several tens of ppm throughdegassing in a steelmaking process. In such a technology, drawabilityand aging resistance are improved by adding Ti, Nb, etc. to avoid theadverse effects of solute C or solute N that still remains slightly in asteel sheet. However, the problems of the technology are that thedefects such as bubbles and black spots are likely to be caused bycarbides and nitrides and the production cost increases due to theaddition of Ti, Nb, etc.

As technologies of solving the problems, steel sheets for vitreousenameling, wherein the addition amounts of Ti, Nb, etc. are decreased,though drawability deteriorates to some extent, and methods forproducing the steel sheets have been invented and are disclosed inJapanese Unexamined Patent Publication Nos. H8-27522 and H10-102222 andother publications. These technologies are ones wherein B is usedpredominantly for fixing N. However, the problems of the aforementioneddisclosed technologies are: that aging properties deteriorate and thusformability in pressing is impaired since the lowering of solute C isinsufficient sometimes depending on the production conditions and Nincreases caused by the re-melting of nitrides during annealing; andthat the defects such as bubbles and black spots are likely to be causedby the gases generated by the decomposition of nitrides and the likeduring the baking of a vitreous enamel.

The object of the present invention is to overcome the above-mentionedproblems of a conventional steel sheet for vitreous enameling, toprovide a non-aging and low-cost steel sheet for vitreous enameling, thesteel sheet being excellent in resistance to bubbles and black spots,and to provide a method for producing the steel sheet.

DISCLOSURE OF THE INVENTION

The gist of the present invention is as follows:

(1) A steel sheet for vitreous enameling excellent in workability, agingproperties and enameling properties, said steel sheet containing, inmass,

-   -   C: 0.0050% or less,    -   Si: 0.50% or less,    -   Mn: 0.005 to 1.0%,    -   P: 10×(B−11/14×N) to 0.10%,    -   S: 0.080% or less,    -   Al: 0.050% or less,    -   N: 0.0005 to 0.020%,    -   B: 0.60×N to 0.020%, and    -   O: 0.002 to 0.0800%.

(2) A steel sheet for vitreous enameling excellent in workability, agingproperties and enameling properties, said steel sheet containing, inmass,

-   -   C: 0.0025% or less,    -   Si: 0.050% or less,    -   Mn: 0.10 to 0.50%,    -   P: 10×(B−11/14×N) to 0.030%,    -   S: 0.030% or less,    -   Al: 0.010% or less,    -   N: 0.0035 to 0.0060%,    -   B: 0.60×N to 0.0060%, and    -   O: 0.005 to 0.0450%.

(3) A steel sheet for vitreous enameling excellent in workability, agingproperties and enameling properties, said steel sheet containing, inmass,

-   -   C: 0.0025% or less,    -   Si: 0.050% or less,    -   Mn: 0.10 to 0.50%,    -   P: 10×(B−11/14×N) to 0.030%,    -   S: 0.030% or less,    -   Al: 0.010% or less,    -   N: 0.0005 to 0.0033%,    -   B: 0.60×N to 0.90×N %, and    -   O: 0.005 to 0.0450%.

(4) A steel sheet for vitreous enameling excellent in workability, agingproperties and enameling properties according to any one of the items(1) to (3), said steel sheet further containing one or more of Nb, V,Ti, Ni, Cr, Se, As, Ta, W, Mo and Sn at 0.030 mass percent or less intotal.

(5) A steel sheet for vitreous enameling excellent in workability, agingproperties and enameling properties according to any one of the items(1) to (4), said steel sheet satisfying the following expression:(the amount of N existing as BN)/(the amount of N existing as AlN)≧10.0.

(6) A steel sheet for vitreous enameling excellent in workability, agingproperties and enameling properties according to any one of the items(1) to (5), said steel sheet satisfying the following expression:(the amount of N existing as BN)/(N content)≧0.50.

(7) A steel sheet for vitreous enameling excellent in workability, agingproperties and enameling properties according to any one of the items(1) to (6), wherein, with regard to simple or compound nitrides, thatcontain B or Al and are 0.02 to 0.50 μm in diameter:

-   -   the average diameter of said nitrides is 0.080 μm or larger; and    -   the proportion of the number of the nitrides 0.050 μm or smaller        in diameter to the total number of said nitrides is 10% or less.

(8) A method for producing a steel sheet for vitreous enamelingexcellent in workability, aging properties and enameling properties,characterized by:

-   -   retaining a slab containing the components according to any one        of the items (1) to (4) in the temperature range from 900 to        1,100° C. (Retained Temperature Range 1) for 300 min. or longer        before commencing hot rolling;    -   thereafter retaining it in a temperature range not less than        50° C. higher than said retained temperature (Retained        Temperature Range 2) for 10 to 30 min.;    -   then cooling it to a temperature range not less than 50° C.        lower than said retained temperature (Retained Temperature        Range 3) at a cooling rate of 2° C./sec. or less;    -   retaining it in Retained Temperature Range 3 for 10 min. or        longer; and    -   thereafter commencing hot rolling.

(9) A method for producing a steel sheet for vitreous enamelingexcellent in workability, aging properties and enameling propertiesaccording to the item (8), characterized by further controlling the timeperiod from the time when the coiling of a hot-rolled steel sheetterminates at a temperature of 700 to 750° C. in a hot-rolling processto the time when the temperature of said steel sheet reaches 550° C. orlower to 20 min. or longer.

(10) A method for producing a steel sheet for vitreous enamelingexcellent in workability, aging properties and enameling propertiesaccording to the item (8) or (9), characterized by:

-   -   commencing hot rolling;    -   after the reduction ratio reaches 50% or more, retaining the        hot-rolled material in the temperature range from 900 to        1,200° C. for 2 min. or longer with the temperature of said        material not lowered to 900° C. or lower; and    -   thereafter commencing the hot rolling again.

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention is hereafter described in detail. In the firstplace, the chemical composition of a steel is explained in detail.

It is known that, in steel, the lower the C content is, the better theworkability is. In the present invention, it is necessary to control Ccontent to 0.0050% or less in order to secure a good aging resistance, agood workability and good enameling properties. The preferable range ofC content is 0.0025% or less. Though it is not necessary to specify thelower limit of C content, the practical lower limit thereof is 0.0005%,as a further reduction of the C content increases the cost ofsteelmaking.

Si is not required to be added intentionally and should be as low aspossible as Si deteriorates enameling properties. In the presentinvention, as the deterioration of enameling properties is insignificanteven with a comparatively high Si content, the upper limit of Si contentis set at 0.50%. A preferable Si content is 0.050% or less, similarly tothe case of a usual steel sheet for vitreous enameling, and a yet morepreferable Si content is 0.010% or less.

Mn is a component that influences enameling properties in combinationwith the amounts of oxygen and S. Mn is also an element which preventshot shortness caused by S during hot rolling and, in a steel of thepresent invention, the steel containing a large amount of oxygen, Mncontent is required to be 0.005% or more. On the other hand, when thecontent of Mn is high, enamel adhesiveness is adversely affected andbubbles and black spots are likely to occur and, therefore, the upperlimit is determined to be 1.0%, preferably 0.1 to 0.5%.

P, when its content is low, coarsens the grain sizes of crystals anddeteriorates aging properties, but the lower limit of the content isdetermined in relation to the contents of B and N. On the other hand,when P content exceeds 0.10%, P not only hardens a material anddeteriorates press workability but also accelerates the rate of picklingduring a post-treatment for enameling and increases smuts that causebubbles and black spots. Therefore, in the present invention, it isspecified that the P content is in the range from 10×(B−11/14×N) to0.10%, preferably from 10×(B−11/14×N) to 0.030%.

S increases the amount of smuts in pickling during a post-treatment forenameling and makes bubbles and black spots tend to occur. Therefore,the content of S is set at 0.080% or less, preferably 0.030% or less.

Al, when too much is contained, makes it impossible to control theamount of O in steel within a regulated range. Further, in the controlof nitrides too, Al nitrides generate gases by the reaction with waterduring the baking of vitreous enamel and tend to cause bubble defects,and therefore, Al is not desirable. For those reasons, the content of Alis restricted to 0.050% or less, preferably 0.010% or less.

N is an important element for controlling the state of BN in the presentinvention. It is preferable that the content of N is as low as possiblefrom the viewpoint of aging properties and resistance to bubbles andblack spots. However, when the content is less than 0.0005%, goodproperties can be obtained even without the B addition that is arequirement in the present invention. Therefore, N content in thepresent invention is set at 0.0005% or more. The upper limit of N isdetermined to be 0.020% in relation to B content that is determinedbased on the relationship with oxygen amount in steel. A preferableupper limit is 0.0050%. Note that, in order to control nitrides to adesirable shape, it is preferable that N content is 0.0035 to 0.0060%,more preferably 0.0005 to 0.0033%.

B is also an important element for controlling the state of BN in thepresent invention. Though it is preferable to contain B as much aspossible in order to control BN in a good state, when it is intended toadd B abundantly, the yield in a steelmaking process tends todeteriorate in the case of a steel according to the present inventionthat contains O abundantly. Therefore, the upper limit of B content isset at 0.020%, preferably 0.0060% or 0.90 times the N content. The lowerlimit thereof is set at 0.60 times the N content.

O has a direct influence on fish scale resistance. It also affectsenamel adhesiveness and resistance to bubbles and black spots incombination with the content of Mn. O content of 0.002% or more isnecessary to exhibit such effects. On the other hand, a high O contentmakes the yield of B addition during steelmaking low, makes a good stateof B nitrides difficult to maintain, and deteriorates workability, agingproperties and resistance to bubbles and black spots. For these reasons,the upper limit of O content is determined to be 0.0800%. Therefore, thecontent of O is set at 0.002 to 0.0800%, preferably 0.005 to 0.0450%.

An important condition of the present invention is the control of thekind and amount of B nitrides and a steel according to the presentinvention should satisfy one of the following expressions:(the amount of N existing as BN)/(the amount of N existing as AlN)≧10.0,and(the amount of N existing as BN)/(N content)≧0.50, preferably,(the amount of N existing as BN)/(the amount of N existing as AlN)≧20.0,and(the amount of N existing as BN)/(N content)≧0.70.The reason is not clear yet, but it is estimated that, fixing N asnitrides, particularly as stable B nitrides that are thought to behardly decomposable during annealing or during the baking of vitreousenamel, is effective in securing aging resistance and resistance tobubbles and black spots. Here, (the amount of N existing as BN) and (theamount of N existing as AlN) are the values obtained by analyzing B andAl amounts in a residue when a steel sheet is dissolved in an iodinealcohol solution and then calculating N amounts regarding the whole Band Al amounts as constituents of BN and AlN, respectively.

The distribution of the sizes of nitrides is also an important factorfor improving aging resistance and resistance to bubbles and blackspots. With regard to simple or compound nitrides, that contain B or Al,0.02 to 0.50 μm in diameter, the present invention restricts the averagediameter of the nitrides to 0.080 μm or larger and the proportion of thenumber of the nitrides 0.050 μm or smaller in diameter to the totalnumber of said nitrides to 10% or less. The reason is not clear yet, butit is thought that B nitrides, though they are stable in the state of ahigh temperature such as in the process of annealing or vitreous enamelbaking, are likely to decompose when they are fine and thereforedeteriorate aging resistance and resistance to bubbles and black spots.Here, the number and diameter of the precipitates are the valuesobtained by observing an extraction replica obtained from a steel sheetby the SPEED method using an electron microscope and measuring thenumber and diameter of the precipitates in a visual field not havingdeviation. The distribution of the sizes of the precipitates can beobtained by photographing several visual fields and applying imageanalysis to the photographs. The reason why the diameter of theobjective BN is determined to be 0.02 μm or larger is that thequantitative and qualitative analyses of fine precipitates are not saidto be perfect even with the latest measurement technology and thus largeerrors may occur. Further, the reason why the diameter of the objectivenitrides is determined to be 0.50 μm or smaller is that, when B, Al or Nis contained in large oxides that are contained abundantly in a steelaccording to the present invention, it may also be measured undesirablyand may create errors in the measurement results of the objectivenitrides. For those reasons, in the present invention, the range ofnitrides is specified in relation to the precipitates having the sizesthat allow yet smaller measurement errors to be expected. A precipitatewhose shape is elongated is sometimes observed particularly among theprecipitates that are compounded with MnS. In such a case, where theshape is not isotropic, the average of the length and breadth is used asthe diameter of the precipitate.

It is well known that Cu has the functions of suppressing the rate ofpickling during a post-treatment for enameling and of improvingadhesiveness. To add Cu by about 0.02% in order to effectuate thefunctions of Cu in a one-coat enameling treatment does not hinder theeffects of the present invention. However, the amounts of solute C and Nare very small in case of the present invention, and therefore, when thefunction of suppressing pickling is excessively strong, the adhesivenessin the duration of low pickling deteriorates. For that reason, the upperlimit of Cu content should be restricted to about 0.04% even when Cu isadded.

Ti, Nb, V, Ni, Cr, Se, As, Ta, W, Mo and Sn do not hinder the effects ofthe present invention as long as one or more of them are contained at0.030% or less in total. In other words, as long as the total content ofthem is within aforementioned range, it is possible to add themactively, in addition to such an amount thereof as to be unavoidablyincluded from iron ore, scraps and others, with the expectation that theadvantages in a production method or in quality, other than theadvantages envisaged in the present invention, may be obtained.

Next, the production method is explained hereunder. The effects of thepresent invention can be obtained under any of the casting methods.

A temperature history during hot rolling largely affects the control ofB precipitates as described above. In order to control the value of (theamount of N existing as BN)/(the amount of N existing as AlN) to 10.0 ormore, it is desirable, for example: to retain a slab in the temperaturerange from 900 to 1,100° C. (Retained Temperature Range 1) for 300 min.or longer before commencing hot rolling; thereafter to retain it in atemperature range not less than 50° C. higher than the retainedtemperature (Retained Temperature Range 2) for 10 to 30 min.; then tocool it to a temperature range not less than 50° C. lower than theretained temperature (Retained Temperature Range 3) at a cooling rate of2° C./sec. or less; to retain it in Retained Temperature Range 3 for 10min. or longer; and thereafter to commence hot rolling.

On the other hand, it is also possible to control the state of Bprecipitates by a temperature history after hot rolling.

In order to control the value of (the amount of N existing as BN)/(Ncontent) to 0.50 or more, it is desirable, for example, to control thetime period from the time when the coiling of a hot-rolled steel sheetterminates at a temperature of 700 to 750° C. to the time when thetemperature of said steel sheet lowers and reaches 550° C. or lower to20 min. or longer.

Further, it is possible to optimize the distribution of the sizes ofprecipitates by controlling a temperature history and a reduction ratioduring hot rolling.

In order to satisfy the condition that, with regard to simple orcompound nitrides, that contain B or Al, 0.02 to 0.50 μm in diameter:the average diameter of said nitrides is 0.080 μm or larger; and theproportion of the number of the nitrides 0.050 μm or smaller in diameterto the total number of said nitrides is 10% or less, it is desirable,for example: to commence hot rolling; after the reduction ratio reaches50% or more, to retain the hot-rolled material in the temperature rangefrom 900 to 1,200° C. for 2 min. or longer with the temperature of saidmaterial not lowered to 900° C. or lower; and thereafter to commence thehot rolling again.

That is, the object of specifying hot-rolling conditions as explainedabove is to control the shape of precipitates in a desirable state.

The higher the temperature before the commencement of hot rolling is,the more the precipitates dissolve. Then, as the temperature lowers withthe progress of the succeeding hot rolling, the possibility thatdissolved elements may precipitate at undesirable element ratios or inundesirable shapes increases.

If the temperature lowers excessively, not only cannot the compositionratio of precipitates be controlled in a preferable state but also thedispersion of precipitate-forming elements during the retention of thetemperature is decelerated, and that makes the growth of theprecipitates not as expected.

Considering the growth of precipitates during the retention of thetemperature in particular, it is necessary to take the influence of notonly the temperature but also the time into consideration. The controlof a cooling rate is important in order to suppress the fining of theprecipitates that are formed as the elements having dissolved during theretention of the temperature precipitate with the drop of thetemperature.

It is desirable to strictly control a heat pattern including a heatingtemperature, a heating time and a cooling rate in order to ideallycontrol precipitates.

Further, with respect to precipitation behavior, a precipitationpromotion phenomenon (strain induced precipitation) caused byintroducing a strain during the precipitation is well known, and whenthe strain induced precipitation is applied to a steel according to theresent invention, the composition ratio of the precipitates becomes in apreferable state. The reason is not clear yet, but it is estimated that:a strain caused by the consistency with a parent phase varies with thekind of precipitates; therefore the interaction with the work inducedstrain also varies with the precipitates; and thus, in a steel accordingto the present invention, the precipitates preferable to workability andaging properties grow preferentially.

The aforementioned temperature control is applied in the state wherein aparent phase of a steel sheet is predominantly composed of an austenitephase, and the temperature history after a parent phase has transformedinto ferrite due to a temperature drop at the latter half stage of ahot-rolling process is also important.

It is thought that the reason is that, in the present invention, thoughthe solubility of the main objective precipitates may decrease with theparent phase transforming from austenite to ferrite and theprecipitation may proceed rapidly, stable precipitates vary with theparent phase.

That is, as the precipitates that have been stable up to that time aredecomposed and new precipitates that are newly stabilized are formed bythe transformation of a parent phase, the composition of theprecipitates varies consecutively.

From this viewpoint, the temperature history in a coiling processwherein a steel sheet is retained at a relatively high temperature in aferrite phase is important.

It is desirable that the cold reduction ratio is 60% or more in order toobtain a steel sheet having a good drawability. When a yet higherdrawability is required in particular, it is preferable that the coldreduction ratio is 75% or more.

With respect to annealing, the effects of the present invention do notchange with either box annealing or continuous annealing and they areexhibited as long as the temperature is not lower than therecrystallization temperature. From the viewpoint of the cost reductionthat is a feature of the present invention in particular, continuousannealing is preferable. A steel according to the present invention isnot necessarily annealed at a high temperature since it is characterizedby completing recrystallization at 630° C. even with short-timeannealing.

Skin-pass rolling is carried out for the purpose of correcting the shapeof a steel sheet or suppressing generating yield-point elongation duringworking. Skin-pass rolling is applied usually at the reduction ratio ofabout 0.6 to 2% in order to suppress yield point elongation while thedeterioration of workability (elongation) due to rolling working isavoided. However, in the present invention, the generation of yieldpoint elongation is suppressed even without the application of skin-passrolling, and the deterioration of workability is low even with arelatively high reduction ratio in skin-pass rolling. When skin-passrolling is applied, it is desirable to set the range of the reductionratio at 5% or lower.

Further, in order to secure enamel adhesiveness, it is preferable, forexample, to apply Ni plating of about 0.01 to 2 g/m² after cold rollingor after annealing.

EXAMPLE

The continuously cast slabs consisting of the various chemicalcompositions shown in Table 1 were subjected to hot rolling, coldrolling, annealing and skin-pass rolling under the conditions shown inTable 2. The state of nitrides, mechanical properties and enamelingproperties of the steel sheets are shown in Table 3.

The mechanical properties were evaluated by the tensile tests specifiedin JIS No. 5 Test. An aging index (AI) was obtained by imposing aprestrain of 10% with a tension and measuring the difference of thestresses between before and after the aging at 100° C. for 60 min.

The enameling properties were evaluated after the process steps shown inTable 4. Among the enameling properties, the surface properties ofbubbles and black spots were evaluated by the visual observation underthe condition of a long pickling time of 20 min. The enamelingadhesiveness was evaluated under the condition of a short pickling timeof 3 min. Because the commonly employed P.E.I. adhesiveness test method(ASTM C313-59) was incapable of detecting small difference in the enameladhesiveness, the enamel adhesiveness was evaluated by dropping a 2.0-kgweight with a spherical head on a test piece from a height of 1 m,measuring the exfoliation state of the enameling film at the deformedarea using 169 probing needles, and calculating the percentage of thenon-exfoliated area. The fish scale resistance was evaluated by carryingout the accelerated fish scale test, wherein three steel sheets werepre-treated through 3-min. pickling without Ni immersion, glazed with aglaze for direct one-coat enameling, dried, baked for 3 min. in a bakingfurnace kept at 850° C. and having a dew point of 50° C., and then heldfor 10 h. in a constant temperature tank kept at 160° C., and byvisually judging the occurrence or otherwise of fish scales.

As is clear from the results shown in Table 3, the steel sheetsaccording to the present invention are the steel sheets for vitreousenameling excellent in workability (elongation), aging resistance andenameling properties.

[Tables 1 to 4]

TABLE 1 Chemical components (mass %) No. C Si Mn P S Al N B O 1 0.00200.011 0.32 0.009 0.018 0.002 0.0034 0.0023 0.041 2 0.0012 0.45 0.150.088 0.012 0.003 0.0029 0.0020 0.035 3 0.0008 0.005 0.30 0.002 0.0110.001 0.0032 0.0025 0.062 4 0.0015 0.008 0.26 0.041 0.015 0.002 0.00450.0070 0.026 5 0.0016 0.005 0.28 0.015 0.026 0.005 0.0115 0.0075 0.033 60.0022 0.003 0.44 0.007 0.052 0.002 0.0035 0.0033 0.044 7 0.0027 0.390.80 0.023 0.015 0.003 0.0020 0.0024 0.008 8 0.0018 0.006 0.30 0.0830.015 0.006 0.0022 0.0034 0.010 9 0.0016 0.004 0.33 0.012 0.015 0.0080.0041 0.0028 0.007 10 0.0011 0.022 0.06 0.006 0.011 0.002 0.0052 0.00390.039 11 0.0006 0.005 0.26 0.019 0.023 0.001 0.0049 0.0055 0.024 120.0020 0.004 0.27 0.004 0.024 0.005 0.0057 0.0041 0.011 13 0.0014 0.0020.38 0.026 0.016 0.003 0.0050 0.0040 0.034 14 0.0012 0.005 0.04 0.0280.014 0.004 0.0042 0.0057 0.036 15 0.0035 0.008 0.22 0.010 0.011 0.0030.0049 0.0033 0.029 16 0.0013 0.009 0.17 0.009 0.008 0.003 0.0054 0.00340.035 17 0.0012 0.004 0.12 0.009 0.002 0.002 0.0016 0.0013 0.022 180.0011 0.031 0.27 0.008 0.015 0.007 0.0032 0.0027 0.015 19 0.0016 0.0050.30 0.004 0.012 0.002 0.0018 0.0013 0.026 20 0.0011 0.005 0.35 0.0220.011 0.003 0.0020 0.0017 0.035 21 0.0022 0.003 0.30 0.010 0.019 0.0080.0028 0.0022 0.008 22 0.0014 0.004 0.24 0.010 0.022 0.004 0.0022 0.00180.031 23 0.0018 0.005 0.18 0.009 0.015 0.002 0.0023 0.0016 0.042 240.0010 0.003 0.21 0.013 0.014 0.004 0.0020 0.0013 0.030 25 0.0060 0.0030.32 0.015 0.012 0.003 0.0034 0.0015 0.031 26 0.0022 0.003 0.35 0.0150.008 0.012 0.0016 0.0032 0.001

TABLE 2 Hot-rolling heating conditions Cooling rate from Retained Tem-perature Hot-rolling conditions Retained Retained Range 1 to RetainedReduction Temperature Temperature Retained Temperature Hot-rolling ratioRange 1 Range 2 Tem- Range 3 coiling conditions before Reten- Tem- Tem-perature Tem- Coiling Retention temperature Retained tion perature Timeperature Time Range 2 perature Time temperature time retentiontemperature time No. (° C.) (min.) (° C.) (min.) (° C./sec.) (° C.)(min.) (° C.) (min.) (%) (° C.) (min.) 1 1100 250 — — — — — 650 10 — — —2 1080 360 1150 15 1.5 1030 30 650 10 — — — 3 1080 360 1150 15 1.5 103030 650 10 — — — 4 1080 360 1150 15 1.5 1030 30 650 10 — — — 5 1080 3601150 15 1.5 1030 30 730 100 — — — 6 1080 360 1150 15 1.5 1030 30 730 100— — — 7 1080 360 1150 15 1.5 1030 30 730 100 — — — 8 1080 360 1150 151.5 1030 30 730 100 75 950 10 9 1100 250 — — — — — 650 10 — — — 10 1080360 1150 15 1.5 1030 30 650 10 — — — 11 1080 360 1150 15 1.5 1030 30 65010 — — — 12 1080 360 1150 15 1.5 1030 30 650 10 — — — 13 1080 360 115015 1.5 1030 30 730 100 — — — 14 1080 360 1150 15 1.5 1030 30 730 100 — —— 15 1080 360 1150 15 1.5 1030 30 730 100 — — — 16 1080 360 1150 15 1.51030 30 730 100 75 950 10 17 1100 250 — — — — — 650 10 — — — 18 1080 3601150 15 1.5 1030 30 650 10 — — — 19 1080 360 1150 15 1.5 1030 30 650 10— — — 20 1080 360 1150 15 1.5 1030 30 650 10 — — — 21 1080 360 1150 151.5 1030 30 730 100 — — — 22 1080 360 1150 15 1.5 1030 30 730 100 — — —23 1080 360 1150 15 1.5 1030 30 730 100 — — — 24 1080 360 1150 15 1.51030 30 730 100 75 950 10 25 1100 250 — — — — — 650 10 — — — 26 1100 250— — — — — 650 10 — — — The symbol “—” shows that the condition was notapplied.

TABLE 3 Average grain Proportion of Mechanical Enameling diameter fineproperties Aging properties properties NasBN/ (μm) precipitates in TS EIAI Fish scale Adhesiveness Surface No. NasAlN NasBN/N in claim 7 claim 7YP (MPa) (MPa) (%) (MPa) resistance (%) properties Remarks 1 5.2 0.430.02 90 186 316 54 30 Δ 95 Δ Invented 2 3.3 0.20 0.09 10 375 523 33 50 ◯92 Δ examples 3 6.9 0.80 0.05 60 177 308 56 5 ◯ 92 Δ 4 12 0.48 0.05 70251 385 48 30 ◯ 93 Δ 5 14 0.52 0.07 20 217 348 51 5 ◯ 90 ◯ 6 8.8 0.900.10 7 200 330 50 1 ◯ 94 ◯ 7 16 0.33 0.12 8 295 435 25 20 ◯ 90 ◯ 8 260.82 0.14 8 339 477 39 2 ◯ 100 ⊚ 9 6.3 0.45 0.05 20 204 335 52 2 ◯ 96 ◯10 6.8 0.45 0.24 5 160 291 60 2 ◯ 98 ◯ 11 6.8 1.00 0.19 40 209 341 52 0⊚ 96 ◯ 12 21 0.37 0.24 15 171 301 55 10 ⊚ 96 ◯ 13 >50 0.90 0.12 80 220353 49 1 ⊚ 98 ⊚ 14 6.7 0.95 0.35 4 235 368 46 1 ◯ 100 ◯ 15 24 0.77 0.282 190 320 54 3 ⊚ 98 ⊚ 16 >50 1.00 0.55 2 179 309 56 0 ⊚ 100 ⊚ 17 4.10.22 0.08 30 171 302 59 10 ◯ 95 Δ 18 8.2 0.48 0.19 10 197 328 53 8 ◯ 98◯ 19 6.7 0.74 0.06 50 166 296 56 6 ◯ 98 ◯ 20 >50 0.48 0.14 20 204 336 527 ◯ 95 ◯ 21 25 1.00 0.06 20 205 336 45 0 ◯ 100 ◯ 22 9.1 1.00 0.13 5 179310 56 2 ◯ 100 ◯ 23 14 0.41 0.31 5 181 312 57 5 ⊚ 98 ⊚ 24 >50 0.83 0.144 182 314 56 0 ⊚ 100 ⊚ 25 5.3 0.20 0.04 80 222 322 53 70 X 85 XComparative 26 8.1 0.43 0.07 30 212 312 50 40 X 80 X examples ⊚: Verygood, ◯: Good, Δ: Conventional level, X: Poor

TABLE 4 Process steps Conditions 1 Degreasing Alkaline degreasing 2 Hotwater rinse 3 Water rinse 4 Pickling 15% H₂SO₄, 75° C. × 3 or 20 min.immersion 5 Water rinse 6 Ni treatment  2% NiSO₄, 70° C. × 3 min.immersion 7 Water rinse 8 Neutralization  2% Na₂CO₃, 75° C. × 5 min.immersion 9 Drying 10 Glazing Direct one-coat glaze, 100 μm in thickness11 Drying 160° C. × 10 min. 12 Baking 840° C. × 3 min.

The steel sheets according to the present invention have goodworkability and further satisfy all of the fish scale resistance, enameladhesiveness and surface properties that are required of a steel sheetfor vitreous enameling. In particular, the present invention makes aconsiderable cost reduction possible and has a great industrialsignificance, because it makes it viable to produce a steel sheetexcellent in workability and aging resistance with decarbonizationannealing or decarbonization and denitrification annealing that can beapplied to a conventional high-oxygen steel without containing expensiveelements such as Ti or Nb.

1. A steel sheet for vitreous enameling excellent in workability, agingproperties and enameling properties, the steel sheet comprising: no morethan 0.0025 mass percent carbon; no more than 0.010 mass percentsilicon; from 0.1 to 0.5 mass percent manganese; 0.0005 to 0.0033 masspercent nitrogen; from 0.60 times the amount of nitrogen to 0.0060 masspercent boron; from 10 times the difference between the amount of boronand 11/14 times the amount of nitrogen, 10×(B−11/14×N), to 0.030 masspercent phosphorous; no more than 0.030 mass percent sulfur; no morethan 0.010 mass percent aluminum; 0.005 to 0.0450 mass percent oxygen;and a balance of Fe and unavoidable impurities, wherein the steel sheetcontains simple or compound nitrides, having a diameter of 0.02 to 0.50μm, containing boron or aluminum, and having an average diameter of atleast 0.080 μm, and the proportion of the number of the nitrides of0.050 pm or smaller in diameter to the total number of the nitrides is10 percent or less, wherein the ratio of the amount of nitrogen in thesteel sheet existing as boron nitride, BN, to the amount of nitrogen inthe steel sheet existing as aluminum nitride AlN,(the amount of N existing as BN)/(the amount of N existing as AlN), is≧10.0; and the steel sheet further comprises a nickel plating on thesteel sheet in an amount of about 0.01 to 2 g/m².
 2. The steel sheet forvitreous enameling according to claim 1, wherein the steel sheet furthercomprises one or more of niobium, vanadium, titanium, nickel, chromium,selenium, arsenic, tantalum, tungsten, molybdenum, and tin, each in anamount of no more than 0.030 mass percent.
 3. A method for producing asteel sheet for vitreous enameling excellent in workability, agingproperties and enameling properties and enameling propertiescharacterized by: retaining a slab containing the components accordingto claim 1, in the temperature range from 900 to 1,100° C. (RetainedTemperature Range 1) for 300 minutes or longer before commencing hotrolling; thereafter retaining it in a temperature range not less than50° C. higher than said retained temperature (Retained Temperature 2)for 10 to 30 minutes; then cooling it to a temperature range not lessthan 50° C. lower than said retained temperature (Retained Temperature3) at a cooling rate of 2° C./sec. or less; retaining it in saidretained Temperature 3 for 10 minutes or longer; and thereaftercommencing hot rolling.
 4. A method for producing a steel sheet forvitreous enameling excellent in workability, aging properties andenameling properties and enameling properties according to claim 3,wherein hot-rolling is controlled under the condition of the time periodfrom the time when the coiling of a hot-rolled steel sheet terminates ata temperature of 700 to 750° C. in a hot-rolling process to the timewhen the temperature of said steel sheet reaches 550° C. or lower for 20minutes or longer.
 5. A method for producing a steel sheet for vitreousenameling excellent in workability, aging properties and enamelingproperties and enameling properties according to claim 3, wherein thehot-rolled steel sheet is retained in the temperature range from 900 to1,200° C. for 2 minutes or longer with the temperature of said steelsheet not lowered to 900° C. or lower when the reduction ratio reaches50% or more after commencing hot-rolling, and thereafter hot-rolling iscommenced again.